The present invention relates to a rotor used for a permanent magnet rotating machine comprising the rotor comprising a plurality of permanent magnets embedded in a rotor core and a stator formed by winding a winding wire through a stator core having a plurality of slots, the rotor and the stator disposed with a clearance interposed therebetween (which the machine is so-called a magnet-embedded rotating machine or an IPM (interior permanent magnet) rotating machine), and particularly to a rotor adapted for a permanent magnet rotating machine best suited for a large wind power generator and the like.
An Nd-based sintered magnet is used for more and more various purposes because of its excellent magnetic properties. In recent years, in a field of rotating machines such as a motor and a generator, permanent magnet rotating machines comprising the Nd-based sintered magnets have been developed as devices are reduced in weight, thickness, length, and size, improved in performance, and become more energy-efficient. Because it is possible to utilize reluctance torque by magnetization of a rotor yoke in addition to torque by magnetization of magnets in an IPM rotating machine having the magnets embedded in a rotor, the machine has been studied as a high-performance rotating machine. Because the magnets are embedded in the rotor yoke of silicon steel sheets or the like, the magnets do not jump out due to centrifugal force during rotation. Accordingly, the rotating machine has high mechanical safety, can be operated with high torque and at a wide range of rotation speed by controlling a phase of current, and can be an energy-efficient, highly efficient, and high-torque motor. In recent years, the applications such as a motor or a generator for an electric car, a hybrid car, a high-performance air conditioner, industrial purposes, a train and the like have been rapidly expanding.
For the future, the Nd-based sintered magnet is expected to be applied to a large wind power generator. Higher efficiency, increase in an electric generating capacity, improvement of electric power quality, and a lower failure rate are required of the large wind power generator. For this reason, use of the rotating machine comprising the Nd-based sintered magnet for wind power is expected to expand rapidly for the future.
In general, the permanent magnet in the rotating machine is apt to be demagnetized by the action of diamagnetic fields due to the winding wire so that a magnetic coercive force has to be the same as or higher than a certain value. Because the magnetic coercive force reduces as temperature increases, the magnet having a greater room-temperature magnetic coercive force is required when it is used for high-speed rotation in which heat generation due to an eddy current in the magnet is not ignorable. On the other hand, a residual magnetic flux density which is an index of magnetic force needs to be as high as possible because it directly influences the electric generating capacity.
There is a tradeoff relationship between the magnetic coercive force and the residual magnetic flux density of the Nd-based sintered magnet so that the residual magnetic flux density decreases as the magnetic coercive force increases. Accordingly, there is a problem that the electric generating capacity reduces when the magnet having an unnecessarily high magnetic coercive force are used for the generator.
In recent years, as reported in WO2006/043348A1 and in Kenichi Machida, Takashi Kawasaki, Shunji Suzuki, Masahiro Ito and Takashi Horikawa, “Grain Boundary Reforming and Magnetic Properties of Nd—Fe—B-based Sintered Magnet”, Abstracts of Lectures of Japan Society of Powder and Powder Metallurgy, Spring Meeting in 2004, p. 202, there is a method for increasing a magnetic coercive force without reducing a residual magnetic flux density in which Dy (dysprosium) or Tb (terbium) is diffused from a surface to an inside of a sintered magnet. Because it is possible to efficiently thicken Dy and Tb at a grain boundary by the method, it is possible to increase the magnetic coercive force with little reduction in the residual magnetic flux density. Moreover, the smaller the size of the magnet, the further Dy or Tb diffuses inward so that the method can be applied to a small or thin magnet.
Reported in JP 2008-061333A is a surface magnet rotating machine (so-called a SPM (surface permanent magnet) rotating machine) comprising magnets having been subjected to diffusion of Dy or Tb. It is effective to increase the magnetic coercive force of thin portion of the D-shaped magnet and such a magnet is obtained by diffusing Dy or Tb to the magnet.